This invention relates to hydraulic pressure control valves particularly adapted for use in vehicle braking system.
Conventionally, in applying brakes on a vehicle a major portion of the load of the vehicle, will act on the front wheels and the load acting on the rear wheels will be reduced due to the so-called "nose dive phenomenon", thus, the rear wheels will be locked earlier than the front wheels if the braking force applied to the front and rear wheels are equal and the deceleration of the vehicle exceeds a predetermined value. Therefore, it is required to reduce the braking pressure applied to the rear wheels as compared with that to the front wheels in accordance with the brake applying force and with the deceleration of the vehicle. There is usually provided a hydraulic pressure control valve for controlling the braking pressure applied to the rear wheels, such that the braking pressure for the rear wheels will elevate at a reduced rate as compared with the braking pressure applied to the front wheels.
One prior art hydraulic pressure control valve of the aforementioned kind comprises a valve body mounted on one of end walls of a cylinder, and a piston acting as a control valve member is slidably disposed in the cylinder so that the effective area of a communicating passage formed in the piston is controlled by the valve body in response to the sliding movement of the piston in the cylinder. A spring is disposed between the end wall of the cylinder and the piston to bias the same in the opening direction. For controlling the spring force of the spring in accordance with the deceleration of the vehicle, an inertia valve is provided. Upon depressing a brake pedal the braking pressure in the front wheels moves the piston in the closing direction and cuts off the communication through the communicating passage at a predetermined pressure level (cut-off pressure) thus controlling the braking pressure supplied to the rear wheels. Thereafter, when the pressure supplied to the front wheels further increases, the passage repeatingly opens and closes, and the pressure for the rear wheels increases at a reduced rate as compared with that of the front wheels.
In the prior art pressure control valve, the mechanism for changing the force of the spring has a very complicated construction. Further, the inertia valve usually comprises a ball member rotatably provided on an inclined guide way, and there are problems that the inertia valve closes too early in a down-hill running condition whereby an excessively large braking force will act on the front wheels, and that the inertia valve will sometimes be held open due to dust or the like on or around the valve seat and/or due to an up-hill running condition, and thus the pressure supplied to the rear wheels excessively increases and the rear wheels will be locked earlier than the front wheels.